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Did people really hand-start big bombers in WWII?
I am not a pilot so I don't know anything about hand starting propeller aircraft. But I wonder how easily can the propeller fire up unexpectedly during the procedure even if it's done right. Whenever I see an old WWII movie and the guys are turning those big props on the bombers, I always think of that question.

They are not hand-starting those propellers in the movies/documentaries. They are "running them through" to get the oil out of the lower cylinders.

@Ron Beyer: That should be an answer.

Not a duplicate but this question might have some good answers:  https://aviation.stackexchange.com/questions/33094/is-it-possible-to-hand-start-modern-planes

When you shut down a radial the unscavenged oil in the case (oil coating the surfaces that didn't get pumped back to the reservoir tank) runs down and seeps past the rings of the cylinders directly below and to each side at the bottom.  In theory, it can create an hydraulic lock if there is enough oil collected in a cylinder, that happens to be on its compression stroke (both valves closed), and the oil volume is more than the cylinder volume at top dead centre.  Although an actual hydraulic lock is fairly rare (it takes quite a lot of oil), you still have to check for it because if a jug fires on start while one of the bottom cylinders is locked, it'll bend that jug's connecting rod and/or crack the head.
The engine has to be turned with no ignition/fuel to test for this.  Typically it's done by hand as shown in those films.  If you feel a hydraulic lock coming on as you pull through, you turn the engine the other way to back it off and can remove the lower plugs to let the oil drain out.
An aircraft could have propellers that are out of reach of the ground, or there may be some other reason, so you also have the option of doing it with the starter with fuel and ignition turned off.  If there is an hydraulic lock in a cylinder the starter won't be able to do any damage on its own (it'll either bog down or its clutch will slip), so it's safe to do, if done properly.
There is always some oil collected and when the engine is cranked it goes out the exhaust valve when it opens and coats the inside of the exhaust ducting.  The coating of oil takes quite a while to burn off the inside of the exhaust manifold, which is why they belch smoke for 30 seconds to minute after they start.
It IS possible to "hand start" a big radial in a fix, using things like tires slipped over the prop and pulled by a car with a rope, but not by hand unless it's a smaller radial, say under 1000 cubic inches.

The procedure you seem to refer to isn't hand starting.  It's done on radial engines, with ignition off and valves locked open, to clear oil that may have settled in the lower cylinders (by seeping past the piston rings while the engine was standing) and prevent a hydraulic lock that could bend a piston rod or otherwise break things (at the absolute minimum, foul spark plugs) in the engine if cranked under power without this precaution.
Unless there's something very deeply wrong, the engine can't start with the ignition off.  Engines of the size you'll have seen this done for, however, are never hand started.  Either ground starting carts were used to supply battery power for electric starters, or cartridge starters were used to get the engine spinning so the magnetos could fire the spark plugs.

Could you elaborate on "valves locked open"?

@pericynthion I do not know anything about the specifics of radial engine design, but at least on engines with pushrods and/or rockers, a supplementary lever can push the valve open regardless of the position of the cam (OTOH, you cannot force it closed against the cam.) I have seen diesels where, to start, you pull a long lever which operates a mechanism to open all the exhaust valves, as well as set the fuel pumps to cutoff, and start an electric motor to spin up the engine (easier to do with no compression.) Once it is up to speed, let go the lever and, you hope, it will fire up.

You can't "lock valves open" on a radial or any other piston aircraft engine that I'm aware of.  They can only open from the cam ring's motion as part of the normal cycle.

The precious starting cartridges were the focus of a very dramatic point in "The Flight of the Phoenix". There was no hint that the motor could be started without them; that would have been the 46L R-2800 of the C-82A.

@JohnK Then the reasoning behind clearing the cylinders of oil is completely bogus -- if the bottom cylinder(s) might be on the compression stroke, the engine will hydraulic lock before TDC for that jug.  Pulling the (9, 18, 27) spark plugs, or even just the bottom ones, and then putting them back before the start, isn't practical (takes most of an hour just to put the cowling back on).

Hand-propping was only done on small engines, not bombers or even fighters. It can still be done on small engines today. One of two conditions must be met. Either the ignition must be on. Or, the magnetos are not properly grounded.
Turning a prop past its compression point could cause the engine to fire through as many cycles as the available fuel will allow. Even a few degrees of turning through the arc could cause this to happen.
One of the two magnetos will normally have an impulse coupling on it to assist in spark generation during very low RPMs. Slowly cycling an accidentally ungrounded magneto can cause a horrific accident. This is why you should shut down the engine by starving the engine of fuel. That way, no fuel is left in the cylinders in case of accidental spark.
Also, make sure the mag switch and master switch are off before approaching the front of the aircraft. Lastly, perform a p-lead check just before shut-down.
An automotive example of this danger can be found on old diesel trucks with mechanical, engine-driven fuel pumps. If the fuel shutoff was not engaged, the truck could start itself if it were allowed roll a few feet. Even with no one at the controls.

Larger radials don't use impulse mags.  They use a separate battery powered coil starting circuit in the mag for starting.  One brand, Bendix I think, called it "shower of sparks" iginition.

@JohnK - yet, another reason it was not done on bombers.


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Are there any examples of sustained human-powered flight above ground effect?
Every video of human-powered flight has the plane or helicopter so close to the ground that it benefits from ground-effect. Is there any exception to this rule, i.e. a still-air-level-ground human powered altitude record that is high enough to leave ground effect?

I know paragliders have sometimes achieved impressive altitude records from riding thermals. But that's not exactly human-powered.

The FAI's photo of Daedalus shows its height above the sea to be considerably more than its wingspan, putting it out of ground effect.
(Wikipedia says that most of the flight was 15 to 30 feet high, well within ground effect because the wingspan was 112 feet.)

I couldn't find any official FAI record for height achieved by a human-powered aircraft, though.

It's hard to tell how high it is. Such a light weight plane will not cause any noticeable downwash effects on the water even if it were very low.

It's not the light weight that matters: after all, it also doesn't need much. The low 'downwash effect' is primarily due to high aspect ratio. Or in other words, the ground effect is determined not so much by the wingspan (relative to height) but rather by the wing chord.

This source says the Daedalus never climbed much above 30 feet on its record flight - http://news.mit.edu/1993/daedalus-0106 .  Of course it goes without saying that in photo, the wingpsan appears dramatically smaller than it actually is, due to foreshortening.  The wingspan appears to be not much longer than the longitudinal (tip of nose to tip of tail) length of the aircraft, when in reality it is vastly greater than that.

Yes, some of the prizes awarded to pioneering human-powered aircraft had terms stating specifically that the aircraft must rise above ground effect for some given time period.

More details to be added as I encounter them...

Do you have any sources for your claims?

I fail to see how it answer the question. As i understand, the OP asks for specific examples.

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Can passenger distribution cause a parked airliner to tip?
This question on Travel.SE recounts a situation where a passenger, about to disembark from a commercial airline flight, was asked by a flight attendant to wait for other passengers to "catch up".
This answer suggests that if too many passengers were to disembark from the front, while passengers at the back remained at the back, the plane might tip backward; and that the flight attendant was acting to avoid this possibility.
Is this really a risk on modern airliners, or even regional jets or turboprops?  (The aircraft in the question at hand was a 737.)  Are flight attendants trained to redirect or hold passengers in order to avoid it?
I saw Is passenger weight distribution important on a commercial flight? but it is about the aerodynamic effects of passenger distribution, which are not relevant when the airplane is parked on the ground.

One thing these answers miss is that fact that there have actually been a handful of tail tip incidents on the 737-900. Since it's happened, then the answer is obviously: yes, it's possible. See, for instance this video of a Delta 737 tipping. There was apparently just one at TLV. United warned its pilots about the situation and put in place some procedures to better load cargo and ensure bags are quickly removed from the rear cargo bin before allowing forward passengers off.
To help prevent this, United is rolling out tail stands for these aircraft. Delta has used stands too. This discussion at airliners.net has more from some employees who've used them.

Alaska Airlines uses those tail stands for their 737-900ER planes. I was recently on a United 737-900 and the crew said we couldn't deplane until a certain amount of baggage was unloaded.

To get a better idea of whether this was possible I looked at the weight and balance for a 737. The Airplane Characteristics document provides the necessary information to estimate how the center of gravity (CG) may shift due to passengers deplaning.
The information in the document shows between 89% and 96% of the weight on the main landing gear when the aircraft is within the proper CG range for a 737-800. The max landing weight is about 66,000 kg and the max zero fuel weight is about 62,000 kg, so a weight of 60,000 kg when arriving at the gate seems reasonable for a full flight. If 94% of the weight is on the main gear, this puts the CG at about 1m forward of the main gear, which is towards the aft end but within limits.
I considered the high density layout shown in the document, which is a single-class layout with 175 economy class seats at 32 inch (0.8m) pitch. The first row has just 3 seats and is located about 14m forward of the main gear, with each row of 6 located another 0.8m aft.
I assumed that the aircraft is unloaded from the front, one row at a time, and that each passenger (plus carry-on) weighs 85kg. For a rough idea of the results, consider that the CG starts with a moment of about 60,000 kgm1, and the rows forward of the main gear are an average of 7m forward, so removing about 8600kg from those rows will move the CG aft of the main gear and tip the plane. This means about 100 passengers, which is within possibility for 18 rows of seats.
So with a 60,000 kg aircraft, 94% of the weight on the main gear, and 85kg passengers, the plane will tip after row 13 disembarks. This is very sensitive to CG. If the CG starts at 93% of weight on the main gear, the plane won't tip. If the CG starts fully aft (96% weight on main gear), the plane tips at row 7.
However, the plane isn't really unloaded 1 row at a time. Usually there is a line of passengers down the aisle. If there is 1 passenger per row in the aisle, the plane won't tip at 94% weight on main gear, and won't tip until row 11 at 96% weight on main gear.
A heavier plane will be harder to tip, and heavier passengers will make it easier to tip. Planes that have upper class passengers in the front will be harder to tip, since less weight is removed from the front of the plane. A longer plane (737-900) will actually be slightly harder to tip, as will a shorter aircraft (737-700)2.
So why doesn't this ever happen?
As I mentioned, it's very sensitive to CG. Only an aft CG will be an issue. Also, passengers in the aisle will help balance things out. Many airlines have larger seats with more room in front that will reduce the effect of passengers leaving these rows.
Another factor is baggage. As soon as the plane stops and shuts down the engines, the ground crew will often start unloading the plane. It will take a bit longer to position the jet bridge or stairs and begin disembarking passengers. Removing weight from the rear will help balance passengers leaving the front.
In reference to the post that created this question, it's unlikely that one passenger would make the difference in tipping the aircraft. However, it is entirely possible that fully unloading the seats in front and not the back, and having no one coming down the aisle, is a risk factor that flight attendants would want to avoid.

1: I know that the typical unit is Nm but we only care about balance here, not forces, so there is no need to complicate things with gravity.
2: This may seem strange but for the same weight % (which is how I'm comparing) each aircraft will have a different CG location relative to the main gear, due to the distance between the nose and mains changing. Also, different aircraft have different weights and different CG ranges.

It's possible.
When FedEX used 727s a lot they had regular occurrences of cargo being loaded too far back while the front was empty. The airplane would sit on it's tail. Not usually damaging but having the front door quite a bit higher than usual was an issue. If no one was on board it was awkward to get it back to normal, and FedEX really doesn't like delays. At my airport in the 90s you often saw a post attached to the tail while they ere loading.
Highly unlikely a 747 will rise up like a bucking horse, but it is possible that you can take enough weight off the nose gear of something smaller that the strut can extend. This would make the cabin door rise by about a foot, probably hit the jetway, and upset most everyone onboard. What would the passengers probably do? That's right - run downhill.

Jet bridges can track the door height and automatically raise/lower as necessary (to a point) during loading/unloading.

I must say that a B747 which lost its two rear-centre gear did end up on its tail.

@SMSvonderTann: Yes, but improperly-loaded passengers\cargo do not typically rip off parts of the main landing gear.

A quick search reveals it has happened on cargo planes:

Usually passengers are not that heavy, and there isn't really that much room for 300 people to rush to the back.

Interesting that those both appear to be DC-10s. Was it unusually aft-heavy?

I think the engine in the tail might have something to do with that :)

Do you have any sources for these images?

Hope there wasn't anyone standing under those tails when they tipped...

Most times when tipping is a risk is with smaller aircraft that have the cargo loaded before or after all the passengers in behind the passenger deck. The cargo aircraft you see above are also shifting substantial weight in a very organized fashion. You are gradually moving each container forward, while leaving the other ones behind the main gear.
Tipping over a 737 is not a trivial task I could only find very few occurrences online. Here is why I'm not buying the tipping over argument from the OP at Travel SE:

Tipping over is dangerous for the 100+ passengers still on board. I don't think a lone flight attendant is sufficient integrity to prevent such an event from occurring. If it would be a genuine problem, there would be more measures in place.
The change in weight from the first 5 rows of passengers is pretty insignificant. Ryanair for instance has a habit of even blocking them off entirely when the aircraft is not fully loaded.
There are (very) few recorded instances of it happening despite thousands of 737s in operation.

Most aircraft are designed with bit of weight on the nosewheel anyways to aid steering. If it is too little, it would just slip when they tried to turn.

One possibility is weight and balance related- all jetways have a little 'shoe' that detects if the door is about to contact the jetway floor. If this is the case, it either lowers the jetway or sets of some alarm. The weight shift, while not enough to tip the aircraft over, combined with a high jetway position could have triggered the shoe and they might have waited for the gangway to readjust accordingly.

http://acesflorida.tripod.com/
She might also just have noticed something that required a few seconds of waiting, such as somebody walking up the stairs with prams stored in the cargo hold for the family that just disembarked. Reading up on the OPs comments that she was cabin focused, maybe she just spotted some bag in the luggage rack that was unclaimed and wanted to make sure nobody forgot stuff while deplaning, hence holding up the cabin until it was claimed.

Wouldn't the weight shift have caused the nose to rise, making triggering of the shoe less likely, rather than more?

In the early '90s, I was at the front of the line on the tarmac to board a China Southwest Tu-154 (727ski) in Chengdu, up the aircraft's rear airstair .  We waited a long time and then the line for the front door of the plane boarded.  The miliman managing our line held us up, while watching the front passengers walk up the forward ('58 Buick-like) airstair.  I gave him a questioning glance and he stuck his fingers out like a pistol, then tipped it back with a grin.
This may be only lore, but I was told that the Tu-154 gear was designed a bit farther forward than the 727 because of the larger wheel (rough field) assembly and greater sweep of the 154's wing.

Just add a "yes" as the first word, so this does not get deleted as not answering the question. Besides, the wing of the 154 has the same sweep as that of the 727, but a higher aspect ratio. Tupolev put the main gear in wing pods, which gave them great liberty in placing it. Still, tipping is possible with the 154.

And even more so in the contemporary Il-62 which has an extendable tail support for that very reason.

With the 737-900 tipping is a real issue. IMHO the lack of incidents is down to luck more than anything else.
This was an unloading incident. The rampers can be seen removing bags (weight) from the front pit. Added to this, normal unloading sees all of the 1st class passengers leaving the aircraft followed by economy passengers row by row, front to back. This means at some point all of the passengers in front of the main undercarrage are off the aircraft while all those behind remain on board.
The extra length of the 900 means that the combined weight of the remaining passengers is further back than that of a 700 or 800. When this is added to an empty front cargo/baggage pit tipping is a very real possibility.
The Airlines and Boeing know this problem exists with the 900 and are taking it seriously enough to develop tailstands similar to those used by cargo operators on their 747 freighters.

Source for this?

The reason for this possibility is the way the tricycle landing gear works. In a classic design, the line from the main wheels to the centre of mass (CM) (projected on the vertical plane of symmetry) comes at an angle of 15° forward from vertical. This is only slightly more than the angle to which the aircraft rotates during takeoff. The whole point of this arrangement is that the aircraft could rotate with normal pitch input, and similarly be controllable on landing when touching down on main wheels.
With the CM slightly aft for whatever reason, the airplane will not need much force to tip over, and may remain stable in such position.
Generally, the aircraft with tail mounted engines are more prone to this phenomenon. Most of their payload is located forward of CM (because of the heavy engines on the back), so invariably they have very aft CM when empty. Tu-154 was known to tip over by hand force when empty (typically, when a mechanic was doing some service on the tail).
IL-62 (one of the few airplanes with 4 tail engines) has a special tail wheel to prevent this.
Il-62 support wheel http://cdn-www.airliners.net/aviation-photos/middle/6/3/9/0623936.jpg
Source
P.S. You can find a galore of tipped aircraft of all kinds in this thread (scroll to the middle).

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What is the rationale behind very short distance flights?
I was impressed to read here that some passenger airline routes are even below 60 km in length.
Even assuming that the passengers are already in the airport past the security check, probably a good bus could depart from the same gate and arrive to the destination gate in a very comparable time, as the plane still needs to taxi before and after the flight.
Why these short haul flights are reasonable?

because they have to cross the sea to get to another island?

Buses tend to slow down severely when they drive on the beach, let alone when they enter the sea.

Because air travel is the best option in those places.
The shortest commercial route in the world is between Westray and Papa Westray in the Orkney Islands, a distance of 1.7 miles.
If you see the list of short routes, almost all of them are between islands. That instantly rules out buses and the like unless a bridge is built, which in most cases is not commercially viable. One can use a boat or ferry, but in most cases, the sea may not be deep enough or calm enough.

Any clue how much that costs?

£21 return: http://www.telegraph.co.uk/travel/travelnews/9796008/Bidding-war-for-worlds-shortest-flight.html although that may be subsidised.

I recently flew from Puerto Rico to the VI - we spent more time in security/at the gate than we actually spent in the plane

@mikeTheLiar I recently flew from Paris to Amsterdam on a transfer and I spent more time in security than in the plane. There's a reason people like trains.

Fjords are also a reason these short routes exist, e.g. the trip from KKN to VDS takes less than 10 minutes, vs. driving 165 km (2h 16min according to Google: https://goo.gl/ev5dya)

As an American, I find European countries fascinating. Having just re-read The Hitchhikers Guide to the Galaxy series, I really like fjords right now. :)

Well, you know he got an award for Norway, so if you like fjords you'll love Norway ;-)

@FreeMan Closer to home, Alaska has plenty of fjords, too. (Well, Labrador and Newfoundland is even closer if you're on the east coast.)

@DavidRicherby he., they look even colder than Norway (google "fjord Norway" vs "fjord Alaska"...)

@thebjorn And indeed thy should be, with the Gulfstream running up the coast of Norway

One point not yet mentioned is that most of these short flights are not just between islands, but in archipelagoes. So an aircraft may be dearer to run than a ferry, but the aircraft can do six or seven journeys a day (e.g. Westray to Papa Westray and back, Westray to Mainland, Mainland to Hoy...) and save the costs of three or four ferries.

One other rational for unusually short overland flights is that the short hop is a layover to connect two small airports to a larger one.  A few years ago it was possible to fly between Johnstown PA (JST) and Altoona PA (AOO); which are ~30 miles/60 minutes apart by road.  It wasn't because the airline expected significant amount of traffic between the two towns.  The main route was between Johnstown and Washington-Dulles Airport (IAD) which was flown 3(?) times a day; one of those flights stopped in Altoona with the primary intent of offering a connection from it to DC.  This stop was cancelled in 2014, and only the direct flight is still offered.
Both airports are very small rural locations and commercial traffic is partially funded with Federal Subsidies, so simple economics aren't the only consideration in the route planning.

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How do we use the runway condition assessment matrix?
Starting on October 1st, 2016 the FAA is moving away from providing Mu values for runway friction reports to a new format showing contamination type and depth for each third of the runway.  Along with the report is a code X/X/X showing runway condition codes for each third of the runway.
How do we, as pilots, use the field condition NOTAMs in calculating takeoff and landing performance?

What was wrong with Mu values?

@acpilot - Snoopy, while already tri-lingual, had to learn a fourth language to speak Mu.

I knew it must have been a good reason!


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Where can I find current downloadable tower, ground and approach frequencies for USA airports?
I have searched the FAA site and cannot find a spreadsheet with current freqs for airports, only incomplete lists.  Online flight planning tools must get query-able current freqs on a monthly basis, but I sure couldn't find them.
I'm looking for what Airnav, the Chart Supp US, AOPA flight planner all provide if you search on a specific airport, but in a format I can query from a personal spreadsheet.

Searching for "Airport Database" gave this result...

Didn't think of searching on that phrase... Close, but no cigar.  I am wary of open source data, and I was bummed to find that a navaid local to me (PGY VORTAC) had not had its frequency updated on this site even though the data says it is current as of today.  That means they are not pulling from current info.  Any other good ideas?

Have a look at this question; but it is more about NAVAID frequencies rather than voice frequencies.

I have found something that will work. It is not pretty and I swear I can still smell the punch cards that produced the text files, but here it is:
https://www.faa.gov/air_traffic/flight_info/aeronav/aero_data/NASR_Subscription
It is free, it appears to be updated by the FAA, (I verified it has the current frequency for my local VOR that I know changed 2 months ago) and it is searchable.
Download the current zip file, use the .txt data layout files to determine where in the fixed length record of whatever file your desired data resides in is located.  I did a quick import into Excel and am able to strip the information that I need out based on position in the field.
Not the pretty spreadsheet I was hoping for, but ...
Thanks for the suggestions, everyone. If anyone knows any standard software for the NASR subscription (or what the !@*# NASR stands for), I'd be interested.

Once you get your spreadsheet cleaned up, will you publish it for all of us who are not so enterprising?

@PJNoes, I will try to do that, although since the FAA coughs out an update every 28 days, it would involve me simplifying the spreadsheet merge enough that anybody could do it.  I don't know VBA well enough to make that merge very user friendly, but we'll see.

That information can be found on FAA.gov
You can use the FAA's search tool rather than manually look in a copy of the chart supplement.
However, updated PDF copies of all of the chart supplements are hosted by the FAA here.

Thanks Ryan, but I'm looking for a complete list of all US airports and relevant frequencies that I can download and use offline via my own software. I am familiar with the digital chart supp US options, but pdf's are not searchable by SQL (or at least by  my outdated programming knowledge). Some of the other .csv or .xlsx files don't have all the frequencies I'm looking for.

There are subscription based services by companies like Jeppesen and Lufthansa that provide such data, at a price.
Free data is only available in more cumbersome formats like PDF documents and websites. Handy for looking up individual airports for your trip planning but not for automated data mining.


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If an aircraft is not IFR-equipped nor the pilot IFR rated, is it possible to get a border country to grant access from their CAA authority?
This could also apply to neighbouring countries that are just over water like UK-France, Canada-US (Lakes).
Basically, a VFR flight only with permission granted from that country (if that's possible?).
And if so, then how would it work?
(Obviously you'll need: en route charts, passport, aircraft papers, PPL license.)


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Can a IFR flight plan be amended or cancelled whilst in flight?
I'm asking, because ForeFlight states in their demo training video that an IFR filed flight plans can only be amended or cancelled up to 47 minutes prior to it being filed (and up to 2 hours after, for VFR flight plans that have been filed. Not sure why that's time restricted at 2 hours?).
website: https://foreflight.com/support/video-library/watch/?v=how-to-filing-icao&list=filing-briefing
But in the movies you see jets owners telling the pilots to alternate their course to an alternative location (even International flights).

Don't confuse brand specific software limitations with ATC requirements.

ForeFlight is not connected directly into the ATC system, but submits it through some public interface, so it cannot modify it after it hands it over. But ATC can, it's their system.

Yes, an IFR flightplan can be cancelled at any time. The pilot simply tells ATC "cancelling my IFR flight", then it is cancelled. Depending on airspace classification, ATC may need to issue a clearance to continue as a VFR flight.
However, diverting to another destination than originally planned is not the same as cancelling a flightplan. To do so, the pilot will just inform ATC that they are diverting, and then ATC will provide an appropriate route to fly to get to the new destination. The reporting office at the new destination will, when the flight has landed, send a standard message to the original destination, stating that the flight has landed elsewhere.
Most deviations from your filed flightplan does not actually require an update of the flightplan itself. After all, the flightplan is only the plan you intend to follow at time of departure (or really, at time of filing the plan). Very few flights end up actually following their flightplan to the letter. ATC always keeps a local copy of your flight data, which is updated with any clearances given and other requests that might deviate from your original plan. If the deviation, whatever it may be, affects a downstream ATC sector, it can easily be coordinated verbally. However, if, in some rare case, you need to actually amend your flightplan while in the air, just inform ATC. The controller can either make the required changes (and send related AFTN message) directly, or get in touch with a flight data assistant that can take care of it.

Good answer, recommend changing "would then need" to "may need" regarding the issuance of a VFR "clearance".

Very good answer! I have to add that you can as well ammend your FPL airborne by asking ATC. In some cases it may be possible that eg the aircraft type was filed wrong by accident. After telling ATC, we will call the appropriate AIS units to change the flightplan and distribute the correct version to the respective ATC units

Fun (?) fact, but not an answer to the question and therefore added as a comment: given you and your hardware is rated, you can file an IFR fpl enrout,  should conditions require.

If you look at all the information in a flight plan, the route of flight is a relatively small part of it. Most of the data is about the aircraft (e.g. navigation and safety equipment), crew and passengers, which is used to determine if the flight is legal or by search and rescue folks if the aircraft goes missing. Given the volume of such data and how rarely it changes in flight, once that info is sent to ATC, the only practical way to amend it is to cancel the flight plan and file a new one.
The route of flight, however, is keenly important to ATC and is routinely changed by them for various reasons, both before and after departure. As such, if the pilot wants to change the route (or destination, which would necessarily change the route), that request needs to be made directly with ATC rather than by amending the filed plan.

"the only practical way to amend it is to cancel the flight plan and file a new one." Or just ask ATC to update it for you.

Agree. We amend flight plans every day in the Center, often when the pilot doesn't one one!

@atc_ceedee Toy can seriously amend the non-ATC details like color, survival equipment, etc. without calling FSS?

Yes. All that's necessary from the ARTCC's point of view is a transmission by the pilot, stating the change. As of that moment, it becomes an official record (it's on the tape). For instance, if you jumped in an aircraft at the FBO, but used a flight plan normally reserved for a different aircraft, you could change even your aircraft call sign and type once you're airborne. It's happened, and there are no repercussions.

I've taken full flight plans in this manner, many times. After getting the aircraft type, qualifier, requested route and altitude from a popup, for instance, I can have the pilot give the rest of the flight plan on the frequency, and it becomes a matter of record.

Answering in the context of the faa, as (to my knowledge) ForeFlight is used primarily in the United States:
According to the FAA's ICAO Flight Planning Interface Reference Guide (Version 2.1), Section 2.2.1:

An FPL can be changed by the filing entity until the flight data has been displayed to ATC. This is typically 30 minutes before proposed departure time, but may be an hour or longer in some cases. If a revision is made to a previously filed FPL after the departure flight data has been displayed to ATC, the message will be rejected. If this occurs, call the Flight Data Unit at the departure ARTCC to coordinate the change.

As the paragraph says, at some point (usually 30 minutes prior to the proposed departure time) the flight plan is forwarded to the appropriate ATC unit in the form of a flight progress strip. This is a physical piece of paper which shows only the portion of the flight plan that is directly relevant to ATC: The identifier for the flight, A/C type and equipment information, computer-assigned beacon code, proposed departure time, requested final altitude, departure fix, and route to be flown. Once this flight progress strip has been shown to ATC you are no longer allowed to amend the flight plan yourself because of the risk that ATC might not receive the amendment and then clear you "as filed," leading to a misunderstanding.
However, ATC is able to directly edit all of the above parameters of the flight progress strip if you contact them directly and request it, either before you depart or while you are airborne. So if you (or the VIP in the back) need to change to a different altitude, or change the route you will fly, or divert to another airport entirely, ATC can issue an amended clearance and can update your flight information so the flight data processing computer knows what's going on. You just aren't allowed to send in those amendments yourself.


text:
Why do airplanes circle three times in a holding pattern?
Why do airplanes circle three times in a holding pattern? I tried to look for information about it, but I did not figure out why a pilot would specifically do it three times?

How do you come to the conclusion that they circle three times?

I was asked it as a question by the pilot after I saw one do that, and they did not give me an answer.

Welcome to Aviation! Airplanes will hold as many times as ATC requires. Often it's only once.

I suspect it was a joke, referencing the idea that dogs always turn around three times before lying down.

Now I'm gonna have to watch my dogs for the next week and if I see them turning 2 times, or 4 times, well, you'll be hearing from me!

They don't, necessarily. A holding pattern is a way for air traffic control to slow down an aircraft when an airport does not have enough capacity to meet the number of inbound aircraft (essentially, a traffic congestion). It can also be used in other situations, for example when the weather is too bad for an aircraft to land, and they need to wait for improvement before making an approach.
A standard holding pattern takes 4 minutes to fly. One minute inbound- and outbound legs, and one minutes in each end to turn 180 degrees (a standard turn is 3 degrees per second).

So, if an aircraft needs to be delayed for 12 minutes, 3 turns in a holding is appropriate. However, the actual required delay depends entirely on the situation, and so there is no standard number of turns in a holding pattern - it all depends on what is required.


text:
To which airspace does the border of two adjacent airspaces belong to?
Consider two adjacent airspaces of different categories. For example, airspace G is up to 2500' and above this is an airspace D. The border is at 2500'. Which airspace are you in if flying at exactly 2500'?
I recall a definition by ICAO that said that the border between such airspaces belongs to the less restrictive one, but I can't find that document.

Well, humor aside, changing weather (pressure) makes exact altitude determination difficult.  They don't have fences in the sky.  More importantly, where you are relative to other aircraft and directions of travel take priority.  If I was close, I would abide by the rules of the MORE restrictive one to cover my assets, and follow a flight path that respects the boundary.

The question makes no sense.  Are you really claiming you are flying at exactly 2500'? Improbable.  More likely you are above 2500.000000001 or below 2499.999999999.

@quietflyer In the context, it makes perfect sense. A pilot might be instructed to "maintain 2500 feet", in which case they are considered (for the purposes of ATC instructions, clearances, etc) to be flying at 2500' (as long as instruments show them to be close to that altitude; obviously if they're down at 2200 or whatever then someone will tell them to get back to their flight level). If you've been instructed to fly at 2500', nobody is going to let you off for a violation of airspace restrictions because "my altimeter showed 2499.9999".

In the US, the VFR sectional chart often makes it explicit where the airspace begins and ends.

An example from the SF Bay area. Note that the Class C airspace for Oakland is up to the floor of the Class B for San Francisco. Also note that the Class D airspace for Hayward does not include the floor of the Class C of Oakland.

According to Bold Method, the ceiling of class G airspace extends up to, but does not include, the floor of class D airspace.
At exactly 2500 feet, you are in Class D.  Since radio communication is important, and transponders as well, anywhere close to the boundary may be best erred on the side of caution.
From TheBalanceCareers:  "the pilot must contact the control tower PRIOR to entering
Class D airspace and establish 2-way radio communication" and "you must inform Air Traffic Control of your position, altitude, transponder code, and intentions".  ATC respond with your tail number or call sign to complete 2-way communication. If they don't you can NOT enter.


text:
Why is KLAX and KSEA depicted class D in IFR low enroute chart?
I was looking at ifr low enroute chart and just figured out that KSEA and KLAX are depicted as class D.
Aren't they Class B airports? why do they have D mark?

Does this answer your question? When is this Class D airspace not Class D airspace? and see also: Why does Class D overlap class B around SeaTac?

Related: https://aviation.stackexchange.com/questions/8790/why-does-class-d-overlap-class-b-around-seatac-and-other-seattle-bravo-airsp , https://aviation.stackexchange.com/questions/50109/when-is-this-class-d-airspace-not-class-d-airspace , https://aviation.stackexchange.com/questions/84200/does-this-chart-show-a-heliport-in-class-d-airspace-without-a-control-tower .  I think you'll find your answers there--  (Whoops, guess I hadn't refreshed my window, didn't realize a similar comment was already posted.)

This question appears to be a duplicate of the questions linked above, except for the fact that it specifically addresses the low-altitude IFR enroute charts rather than the VFR sectional charts. Perhaps that's enough to justify a new question?   I'll likely be voting to re-open, if the question gets closed.


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What is the peak of coffin corner called?
The converging point of Vx and Vy is the absolute ceiling. Can I call the peak of the coffin corner an absolute ceiling as well?

The converging point of Vx and Vy is as well though

Yes.
The peak is the actual coffin corner. That's why it is called a corner.
In the coffin corner the aircraft cannot climb any higher, which is a definition for the absolute ceiling. Going any faster will reduce L/D and cause the aircraft to sink; that Mach effects are involved is of minor importance. Going slower will also decrease L/D because the aircraft is approaching stall.
What is special about the coffin corner is not the corner itself, but what happens when the aircraft gets too fast: Now stronger shocks will increase drag and decrease the maximum lift coefficient such that the aircraft is locked into a shallow dive from which it is initially not able to recover. Going too fast while flying at the "regular" absolute ceiling merely means that you will lose some altitude but not lose control.


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Why are some taxiways named with two characters?

The picture above is a diagram of the Taoyuan Airport, Taipei, and as you can see in the diagram, some taxiways are named with two characters like “NC, NP, WC.” My question is: Is there any good reason, if any, why some taxiways are named with two characters when, clearly, there are other single letters still available?

Judging by the airport's layout it stands out that the leading N, S, E, W actually stand for North, South, East, West.

Out of curiosity what's the name of this tablet application?

@kevin FliteDeck Pro

Coding the taxiways this way makes it easy to create mnemonics. Note that from northwest to southeast, the first letters are alphabetically increasing. From southwest to northeast the numbers are increasing.
The combination of a letter and a number gives a good indication of the location of the taxiway.
Every taxiway close to the northern runway (5L-23R) is named Nx, everything close the southern runway (5R-23L) is called Sx. Numbering is from southwest to northeast, so N1 is close to the northern runway, at the west end (5L); S10 is the southern runway close to the eastern end (23L).
Letters can also be combined:
The apron side of the N taxiway is where you park, and called NP.
You cross on the east of the terminal on the eastern crossing or EC, or you cross on the west side of the terminal on WC or W2.

What then are R1-3?

@Pilothead in the alphabet R is just before S and 1 is a low number. I would start my map search just north of the 5R rwy... and there they are.

I'd say the go to the remote stands.

When you see NC (or WC,SC or EC) you would expect a cross taxiway.. eg..north cross. When more than 1 crossing to the north of the terminal then NC1/NC2.


text:
What's the name of the runway equipment referred to in this video?
In this Air Crash investigation video clip (31:06), a beacon system that tells the pilots whether the aircraft is laterally aligned with the designated runway is mentioned. What's its official name?

As this great documentation series also aims for people who are not into aviation, I think the "beacon" just refers to the localizer of the ILS.

It is called the Runway localizer

Oops. I thought it was some special equipment I was not familiar with. Thanks


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What parts of the 787 are produced with 3-D printing (besides air ducts and hinges)?
This article from 2013 states that about 30 parts of the Dreamliner are 3-D printed, mostly air ducts and hinges.
I wonder

what other parts are produced by direct manufacturing?
why it was deemed essential to resort to printing them instead of molding/stamping?

I know this sensor for the GE90 engines was one of the first parts to be approved for use on aircrafts and was 3D printed. This was important since it was a critical engine part. I don't think interior trim parts are held to the same standards (or need to comply with as stringent regulations) as engine components.

why it was deemed essential to resort to printing them instead of molding/stamping?

It may just be a simple case of it being cheaper to 3D print them. Extruding/molding/final machining of complex parts can be time consuming and complicated. If you can 3D print it faster/cheaper or both, than you can make it with traditional methods then printing will win out.
In some cases what may seem like one part could actually be a few parts, bolted, riveted, or welded together. Boeings space division seems to be working on this as well and are able to reduce part complexity by 3D printing parts, according to one of their engineers

“We can build fewer and more complex parts up front, which in turn reduces assembly time and costs,” Wilson said."

For what its worth GE was able to 3D print a whole jet engine recently.

(source)

what other parts are produced by direct manufacturing?

Depending on how you look at it there is an argument for composite structures being in a way direct manufacturing since you are making an entire piece (say a whole wing) in one shot instead of riveting a bunch of sheets of metal to a wing structure or pulling fabric over a steel frame (although you are usually pulling fabric over a foam mold in a sense).

Another example of printed parts is electronics covers.
It wasn't necessarily "essential" to use 3D printing, but economical or beneficial versus other choices. For aircraft parts, 3D printing has a few advantages over more traditional methods of manufacturing:

Much less delay for manufacturing. A new part can be designed and
then 3D printed very quickly. Traditional methods need much more time
to create tooling for each new part. If you are delaying a product
worth millions, time is important, and the faster method will have a
big advantage.
There can also be less cost in manufacturing. The cost of initial
tooling for a part only pays off in the long run if many parts are
produced. A 3D printer can potentially make many different parts,
with minimal added expense to make a new design.
The design can also be made more efficient. 3D printing can create
shapes and designs that are not practical with traditional methods.
This can reduce the weight and complexity of parts.


text:
How should the pressure coefficient distribution from a wind tunnel experiment be interpreted?
I'm doing a wind tunnel experiment and I'm trying to plot the pressure coefficient distribution for the upper and the lower surfaces of an airfoil based on experimental data. The airfoil chord is 150 mm.
I have the pressure static ports located at:
Upper static port #: 1    3    5     7     9     11    13    15     17     19
X-coord [mm]:        0.76 3.81 11.43 19.05 38.00 62.00 80.77 101.35 121.92 137.16

Lower static port #: 2    4    6     8     10    12    14    16    18     20
X-coord [mm]:        1.52 7.62 15.24 22.86 41.15 59.44 77.73 96.02 114.30 129.54

And the pressure readings for each static port are the following:
Static port #:     1      2      3      4      5      6      7      8      9     10    11     12     13    14     15     16    17   18     19     20
Pressure [kPa]:   -0.034 -0.113 -0.087 -0.172 -0.136 -0.168 -0.148 -0.17 -0.15 -0.156 -0.133 -0.15 -0.124 -0.139 -0.12 -0.13 -0.13 -0.13 -0.117 -0.12

I also have Pitot velocity reading: $U_\infty=12.26$ m/s, density: $\rho=1.225$ kg/$m^3$ and $p_\infty=101.3$ kPa.
I know that to compute the pressure coefficient for each static port, I have to use:
$$c_p(x,z(x)) = \dfrac{p(x,z(x))-p_\infty}{1/2\cdot \rho_\infty \cdot U_\infty^2 }$$
I coded a MATLAB script that allows me to compute this equation for the upper part of the airfoil using the odd static ports readings, and for the lower part of the airfoil by using the even static ports readings of pressure.
But I'm having two problems that are a bit strange,

Pressure coefficient must be dimensionless. I have the pressure values in "kPa" and the problem is that if I convert them into "Pa" by multiplying a 1000 factor. The pressure coefficients values will not be around 1, which is what I'm used to seeing in the Cp-chord graphs. How can I fix this? I'm not sure it's correct to use a dividing 1000 factor to compensate this effect.
When I plot the the result I obtain the following result, which is not very similar to the typical Cp-chord plots I have seen in experiments. Am I doing something wrong? Or is it a problem of the experiment in which I obtained bad data?

Could you help me solve my doubts by giving me a little help based on your experience? I'm not asking to get the problem solved, I just want to know how to do it properly.

First, your pressure values cannot be the measured pressure, because they are negative. This indicates that they are pressure differences.
Next, their magnitude is too low to be in Kilopascal. Could it be that you list already the pressure coefficient?
Third, when plotting the pressure coefficient, it is customary to invert the Y axis, so the most negative value is on top. This just makes the plot look more familiar.
If I do this with the tabulated values, this is what I get:

Plot of tabulated values (own work). I extended the Y axis down to +1 on purpose, because this is the stagnation pressure coefficient in in compressible flow. Your data looks like that of a very thin symmetrical airfoil at an angle of attack near 0°.
Now look at this computed plot of pressure coefficients of symmetrical airfoils at zero angle of attack (source):

Inviscid pressure distribution of symmetrical NACA airfoils. The black line looks like your data. Unfortunately you never mentioned which airfoil the measurement is from, but to me it looks awfully like a NACA 0006 (or even something thinner).

Thank you very much! The airfoil is a NACA0012 and the AoA was 0 degrees, as you guessed. And the values I have are literally "readings of static pressure from the ports". So, these are pressure differences? I think I know where was my mistake. Your answer was extremely helpful and elaborated.

@user3780731: The NACA 0012 is the Drosophila of aerodynamics, but due to thickness reaches a minimum $c_P$ of -0.4 at 0°, whereas your data does not even reach -0.2. So these might not be pressure coefficients, after all.

Thank you, very much. Your answer was really helpful. One question, should I assume the Kutta condition in the trailing edge of the airfoil and assume there's a stagnation point at the trailing edge, when plotting upper vs. lower pressure coefficient distribution? (Later I have to compute lift by integrating from leading edge to trailing edge)

@user3780731: Only in inviscid flow - the air needs energy to achieve stagnation pressure, and after being slowed down in the boundary layer, this energy is gone. You will have some recompression, but not nearly as much that you could really speak of a stagnation point. Kutta condition: If no separation occurs, then yes. With separation, the rear airfoil shape matters less; what really counts is the combination of airfoil and momentum thickness

@user3780731 You are very welcome! Just keep asking.


text:
What are the options for renting an aircraft in the Caribbean with an FAA certificate?
I've been looking for places to rent planes in the Caribbean, and haven't been able to find anything that wasn't charter service. Does anyone know of any pilot resources or specific rental or flight schools in the Caribbean, specifically the southern Caribbean?

Renting in the Caribbean seems like a good way to lose aircraft. You know there are smugglers down there that steal boats all the time, then they scuttle them after making their delivery. Also, in the Caribbean aircraft are widely perceived as being magical objects and it is assumed anyone who flies one must be a magical creature worth 100s of millions of dollars. They then charge fees and taxes based on this assumption.

hahaha. Good to know. Does the above logic apply to flying a plane from the US down the island chain?

The Caribbean is big and spans many nations all of whom may have their own aviation regulations. You may want to specify a port of origin for more information.

@Dave I would chose my point or origin based on where I could get an aircraft. But from this, I think I'll just fly down there with a US aircraft.

You would be much better off renting the plane in the United States. It will be difficult to find a rental anywhere in the Caribbean, except maybe Puerto Rico.
You may want to read Greenspun's Flying in the Caribbean. It's a little dated but has good information.

That link is fantastic. Thanks!


text:
Why are there out-of-date tower frequencies on the VFR sectional chart, without a NOTAM?
I flew today to Salinas, CA and had a little trouble reaching Salinas tower (KSNS). The current VFR sectional shows the tower frequency as 119.4. However, apparently the frequency changed to 119.525 on June 2014 (according to http://www.montereybay99s.org/newsletter/July2014issue.pdf)
The current VFR sectional was issued in August 2014, after the frequency change. I'm wondering why (a) the frequency is incorrectly printed on the current VFR sectional, and (b) if it's an error why there is no NOTAM about it.

It's not a printing error. FAA sectional charts are updated only once every six months. In order to to reduce the number of NOTAMs due to sectional changes (which can number in the hundreds between cycles), the FAA provides a section in the Airport Facility Directory (AFD), titled "Aeronautical Chart Bulletin" that identifies changes that have occurred since a chart's publication. Pilots should check this section on each AFD update for changes on their intended route of flight. Once the change is posted in the AFD, the respective NOTAM is taken down.
Here is an image of the applicable AFD page for Salinas, as of the date of this answer:

Thanks, this is great. It's surprising though that the sectional which became active 2 months after the change occurred didn't have the update. You'd think there would've been enough planning time ahead of the freq change to make sure it gets into the next sectional.

@Nils The final image of the chart needs to be ready for print in advance of its release date to allow time to make up the physical copies (and the digital images are identical to the physical ones for obvious reasons: You don't want two people with "the same" sectional to be seeing different data). Of course if the next update cycle doesn't have the correct frequency on it that would be an error, and should be reported so the AeroNav folks can fix it.

do apps like Foreflight include the Aeronautical Chart Bulletin changes?

@rbp I had the same question. On foreflight you can get it in the "southwest a/fd supplemental" in documents tab, faa section. But the actual map in foreflight matches the paper sectional, which is out of date.


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How do they quickly replace the pylons after a hit during the Red Bull Air Races?
My 3 years old was watching the movie Planes (Dusty Crop hopper one) and when he saw one airplane hit a pylon and puncture it he asked: How will they fix it?
I told him it was air filled and they will put a new one there but his question got me wondering as to how can they be replaced quickly before the next guy's run? Is it just like pop one off and put a new one there and quickly blow air with heavy output pumps?

I can't post this as an answer at the moment, so whoever gets to it, here is how the pylons work.

The 25m tall asymmetrical pylons used in Red bull air races are made of 9 different sections, which are connected with zippers for easy removal and installation. In this image, you can see the multiple sections that make up the pylon.

Interior of redbull air race pylons; image from redbull air race
Airgators Team Captain, Holger Leprich, who is responsible for replacing the pylons after a hit, explains the procedure in this video.

The pylons are held up using air pressure- electronically controlled equipment at the bottom of the pylon creates a positive air pressure so that the pylon stands upright.
For each competition, there are three five member pylon replacement teams (called Airgators) to ensure quick replacement- their current record being 67 sec.
The pylon is designed to tear off in case of a hit. Once the pylon is hit, the team moves to the damaged pylons location on clearance from the race director.
Once the team reaches there, a couple of guys control the electronic equipment (and deflate the pylon, if required) at the bottom of the pylon, while another two replace the damaged parts- they simply unzip the damaged part and zip in the new one, while the last one checks the pylon.
Once the replacement is completed, the team inflates the pylon. Mission accomplished.

Link to the photo is now broken and not at that URL in the Wayback Machine.  Do you have an updated link?

From wikipedia

the current structures are made up of six sections attached together by zippers and Velcro to allow quick replacement if damaged by a plane


text:
Why is the maximum speed of current fighter aircraft lower than those from the 1960's and 1970's?
While reading about the characteristics of the newest fighter aircraft, I observed that their maximum speed is lower than of the aircraft in the same roles half a century ago.
For example, the iconic F-14 and Mig-25 have a higher maximum speed than their modern equivalents.
After the Cold War ended, the major air forces are only engaged in ground support and bombing against low-tech opponents who use asymmetrical warfare, and no modern scenario would see the top fighter aircraft directly engaging each other any time soon. However, for any fighter aircraft in the interceptor role, I would expect speed should be an important characteristic. Also, military aircraft are designed and introduced over a long time period, and are expected to be used for many decades.
Why did maximum speed become less important? Wouldn't it be easier to design even faster fighters now than with the technology available 50 years ago?

You're going to have to provide some examples with so many different types on so many roles.

Someone will provide specifics, but right after WWII, the goal was to fly higher and faster so you couldn't be shot down. With the advent of missiles that could go higher and faster than anything manned, the goal became to fly low to avoid detection in the first place. Now the strategy seems to be to fire stand-off weapons to eliminate SAM/AA sites from 50+ miles away, then send in the planes with little threat from the ground.

Added a few examples. Actually, I would be more interested in counter-examples, because most Cold War era fighters could achieve Mach 3, and  many modern fighters can barely achieve Mach 2 (for example, the F-35, even if it is not a pure fighter).

@FreeMan : Still, fighter aircraft do exist, and are developed, having capabilities both for missile and dogfight engagements. By your logic fighters would be completely superfluous. Having a fighter in the role of an interceptor, which is scrambled in the case of an attack, wouldn't it be useful if it could reach the critical area much sooner, or flee much faster if things turn ugly?

Going supersonic to reach the scene burns too much fuel to give you much time on station. Mach 3-4 isn't much use when someone's launched a missile at you that can do Mach 4+. Designing a human-carrying plane capable of such speeds is much more difficult than designing a missile that can achieve those speeds. Agility and ECM are more useful for avoidance than a simple out-run strategy.

SR-71 aside (which isn't a fighter anyway,) what on Earth are you talking about that could exceed Mach 3? F-14 and MiG-29 both have top speeds around Mach 2.3, about the same as the current F-22. The F-15 is a bit faster at Mach 2.5. F-35 is not an air superiority fighter.

According to livescience: MiG-31 M2.83, XB-70 M3.02, Bell X-2 M3.2, MiG-25 M3.2, YF-12 M3.2, SR-71 M3.3, X-15 M6.72. So really, only the 2 MiG's were production fighters capable of near Mach 3. Thus validating both our points, @reirab.

@FreeMan Yeah, I was responding to vsz's assertion that "most Cold War era fighters could achieve Mach 3." The reality is that one Cold War era fighter could achieve Mach 3, as well as the SR-71 and some experimental aircraft.

The entire premise of this question is wrong.  There seems to be little operational difference between 60s, 70s craft and their contemparies in the same roles today.

The maximum speed is lower because of EPA CAFE requirements. ;-)

What about this: SR-72? I think we are going to get some very fast aircrafts soon, but they won't be using normal jet engines

@ThisNameBetterBeAvailable I didn't mention the SR-72 because it wasn't from the Cold War era and because it isn't a fighter (like the SR-71, it's a recon aircraft.)

The MiG-25 that was clocked at M3.2 was in a runaway engine-overspeed state, and the engines were wrecked. Pilots weren't allowed to exceed M2.5 without permission, and the sensible redline speed was M2.8.

@reirab the SR-72  can easily carry a ballistic payload. It can be used for recon and/or strikes.
If only it could fire at full speed...

The SR-72 also suffers from a mild case of non-existence.

@RussellBorogove doesn't get more stealthy than that. It's so stealthy in fact even its owners and users can't see it ;)

Speed was life in air combat until the jet age. The pilots who became Air Force generals in the Fifties had learned their trade in the Thirties when speed was the most-desired quality in a fighter. Naturally, the ability to be faster than any adversary was very important to them. When the requirements for new fighter aircraft were written in the Fifties, those generals made sure that a higher top speed was part of the specification.
When those supersonic-capable aircraft were used in real conflicts, something very surprising and unforeseen happened: They did hardly ever fly supersonic. When the Air Force in the late Sixties accumulated the flight data from several years of Vietnam war air combat, they found that all aircraft had accumulated just minutes at Mach 1.4 and only seconds at Mach 1.6 out of more than 100,000 combat sorties¹. Never was even Mach 1.8 flown in aircraft which had been optimized for Mach 2.4 (F-104, F-105, F-106A, F-4D/E and F-111).
To cite from this study why speed stayed mostly subsonic:

The first of these reasons lies in the shape of the turn rate vs Mach number relationship for an aircraft. […] In combat, each pilot has the tendency to to fly his aircraft so as to maximize his turn rate. He thus gains angular position on the enemy which, in turn, may permit a missile launch or a gun firing. […] It can be seen that the pilot's urge to maximize his turn rate will unfailingly drive his Mach number to about 0.7. Thus, if the pilot is going to join in combat, […] his speed will inevitably drop to subsonic speeds. […]
  Note also, that even if the turn rate is held constant while increasing the speed, […] the turn radius and load factor increase, bringing with it increasing problems of keeping the enemy in sight.

The second reason given in the study is the dramatically smaller combat radius (the maximum distance the airplane can travel from its base, accomplish an objective and return) once the aircraft starts to fly at supersonic speeds. Even for flying into the combat arena supersonic speed was rarely advantageous. Northrop studied a multitude of intercept cases and found that speeds above Mach 1.1 were almost never helpful because they curtailed the combat radius severely.
Now you need to know that the top speed is an important driver in airframe design. Flying at Mach 2+ requires heavy and complex intakes, a heat-resistant structure, high wing sweep and heavy, low-bypass engines. This all degrades the combat qualities at high subsonic speed, which was where those aircraft were used the most. Building into them the capacity for Mach 2+ made them worse for what they were actually used for.
From the late Sixties on this lesson was incorporated into the newer designs like the F-16. Stealth again decreased the importance of supersonic capability, and the maximum sustained speed of the F-22 was actually reduced from Mach 1.8 to Mach 1.6 to reduce the heat load on the composite wing leading edge.
¹ Source: Northrop F-5 case study in aircraft design, AIAA Professional Study Series

Additionally, supersonic speed requires much more fuel.

@GdD: That was one of the reasons why so little time was spent at supersonic speed.

Does the invention of Air to Air and Surface to Air missiles play into this as well?  It seems like no matter how fast and maneuverable you make a plane these days, you can't really outmaneuver or outrun a missile (as opposed to say, another plane with a machine gun.)  Hence the move the stealth and other counter measures rather than relying on airspeed for air superiority.  That factors in as well, no?

@JayCarr: Actually, the first generation of AA missiles misled the generals to believe that dogfighting would not be needed. This made supersonic design easier and the aircraft almost unusable in real life (witness the problems the early F-4 had). Flying supersonic expanded the altitude envelope and helped against early SAMs. Only from the 1970s it became clear that speed was no panacea.

Although it's worth noting that speed is still important for an interceptor: hence maximum speeds remain fairly high. Performance in combat is important, but in some scenarios getting to the enemy fast (eg before they approach your coast) is more important

@JayCarr You can most definitely outmaneuver a missile, or more accurately, you can maneuver to prevent yourself from being in a position which gives the attacker a high probability of kill when they launch the missile. Once launched, if the missile is active or semi-active (ie. it's being guided by the launching aircraft), you can maneuver break the enemy's radar lock. If it's IR homing and not all-aspect, you can maneuver to hide the hottest parts of your aircraft.

@Schwern That's more "maneuvering" in the military sense, I meant it in the aeronautical sense of which craft could withstand higher g forces.  I think, in my head, I'd call what you're talking about "evasion tactics" not maneuvering.  But I see what you're saying.

@JonStory: Even for cases where the interceptor tries to catch up with the target from behind, supersonic flight is a liability because it cuts the mission radius severely. Supersonic capability only helps in combination with big tanks. The F-5 was almost as good as the F-15 in the Northrop studies, and the F-15 scored better mostly due to its size, never actually benefitting from its high top speed.

I'm not saying it's more useful in all circumstances... but if, for example, you have an unresponsive aircraft flying directly toward a Nuclear Site, and your aircraft are 140 miles away (a situation which I believe has happened with Sellafield and RAF Leuchars in the UK) then you'll likely want to get your fighters on scene as fast as physically possible, and worry about fuel later...

Beautiful answer Peter, gems like this are why I come here!

@JayCarr It is quite possible to outrun modern AA or SAM missiles, though not in all circumstances. It's not a matter of speed - it's a matter of position and endurance. If the missile has thirty seconds of fuel, you only have to stay ahead of it for thirty seconds. Even if you would die after a minute, or two minutes, the missile has already lost power (and energy) and is unlikely to be able to maneuver well enough to reach you.

Even the need for speed in the interceptor role has decreased (slightly), as more thorough coverage with better radars is providing more warning time.

The pilots who became Air Force generals in the Fifties had learned their trade in the Thirties when speed was the most-desired quality in a fighter. Please bold that sentence, as it speaks volumes. I'd love to edit it myself but I have a thing against rewriting what other people write.

@DrZ214: Thanks for the compliment! I am hesitant to use boldface in any of my answers; I only do it for structuring the text or to emphasize a single word. Whole boldface sentences are like shouting (in my eyes). Could you forgive me for not following your advice?

Of course I cannot forgive you because there is nothing to forgive :) It is only opinion. IMO, bolding makes people read the most important part and encourages them to read the supporting non-bolded parts too. Bolding makes it more 'colorful' as opposed to blocks and blocks of plain paragraphs that make things seem 'too long.' Bolding helps break it up into easier-to-read chunks. I try to keep it to a minimum of one sentence per paragraph though. BTW, to me shouting is "USING ALL CAPS" or !, which appears more common than bolding! Bolding is a much more professional way to emphasize text, IMO.

The F-106 wasn't used in Vietnam and interceptors like that would still have needed speed in their Cold War role.

Interesting but how does this factor in the supercruise requirement for the AFT / F-22 program?  Was this just intended for use in BVR combat where a stealth fighter can quickly rush an opponent and employ BVR weapons without detection thence quickly escape the lethal envelope of an opponent’s BVR weapons at high speed?

And I also have to wonder if the lower speeds were just a consequence of engine technology combined with the tremendous parasite drag from external weapons carriage.

@CarloFelicione: No. it's the other way around. Engines were optimised for transsonic operations and of course the all-purpose nature of modern jets contributed to that. Not a pound for air to ground was last heard in the late Sixties.

Just for further reference, this new video mentions most of the points in this answer: https://www.youtube.com/watch?v=Xo3arfoGbvc

Because while speed is one of the important characteristics, it is not the only important characteristic (or today, not the most important one) of a combat aircraft.
One important thing to note is that the conditions where top speed is reached is pretty restrictive conditions - high altitude, clean configuration, and afterburners - which are rarely if ever available together in combat.
The evolution of the speeds of combat aircraft in general and combat aircraft in particular were a direct result of the experiences of various air-forces in combat situations over the years.

The main reason for the stress on speed of combat aircraft (at least in initial cases) was due to the lessons learned in the WWII, where extra speed can make the difference between life and death.
After the jets went supersonic, the main reason to have high speeds is for the purpose of interception of enemy aircraft. Speed was the most important requirement in case of combat aircraft (interceptors) half a century ago as the interceptor had to bring the intruder under its range (whether guns or guided missiles). However, the (interceptor) aircraft rarely flew supersonic and as the radars and missiles became advanced, the importance of speed diminished.
Another important reason was to escape from the missiles, especially the surface to air missiles. The idea was that the aircraft can outrun the missile, at least at an altitude. However this was a lost cause from the beginning as the missiles flew far faster than the aircraft. For example, the S300 missiles have a top speed of > Mach 7, more than 2 times that of the aircraft it was designed to shoot down. While the combat aircraft are required to be in service for decades, the missile systems can be developed and fielded much more quickly, erasing any speed gains made. As a result, most of the aircraft initially configured for high speed-high altitude attacks switched over to low altitude penetration tactics.
As the USAF absorbed the lessons learned in the sixties, it became painfully obvious that the combat aircraft were spending less and less time at very high speeds (> Mach 1.5), while most of the combat devolved into close quarter dogfights (in part due to rules of engagement), where maneuverability, rather than speed is more important.
Also, the experiences of the Vietnam and Yom-Kippur Wars, where the western aircraft went against sophisticated Soviet Air defense systems reinforced the fact that the suppression of enemy air defenses is more important than speed.
Designing the aircraft for speed adds significant penalties in the aircraft design- the need for complex air intakes (which adds weight), high temperature materials (which increases cost and maintenance requirements), high powered low-bypass engines (which are usually not fuel efficient). This degrades the other performance parameters of the aircraft in low speed (high subsonic) regime, where most of the operations take place.
As it became clear that the combat aircraft are not going to defeat an air defense system by virtue of speed (or maneuverability for that matter), the strategy again changed- not getting detected by the system in the first place. This emphasis on stealth further reduced the importance of speed (you can only heat the composite airframe so much and afterburners for speed are like lighting a torch in the dark for IR sensors) and enhanced that of the avionics and sensors. Today, the stress is on finding the enemy before getting detected and firing the first shot.
Another reason for high speed is that combat aircraft were also used for reconnaissance, where speed is important (again, for escaping from missiles). However, satellites have taken over most of these duties, further reducing the need for speed.

The epitome of fast combat aircraft is the Soviet Mig-25, which could reach speeds in excess of Mach 3. However, this speed came at a cost- the high temperatures experienced (~300°C) meant that steel had to be used for airframe, requiring complex insulation and cooling systems for avionics and the aircraft rarely flew at this speed due to engine overheating and control issues.
The results (of training and simulation) so far seem to indicate that the modern (fifth generation) aircraft (like F-22) perform better than their fourth generation counterparts (like F-15) in most situations, 'shooting' them down before even getting detected.

One more important point is that the threat model changed as ICBM's replaced bombers as the primary delivery mechanism for nuclear weapons.  Being a few seconds late in an intercept is a much more serious problem if nukes are being thrown around.  Of particular relevance to the OPs question the MiG- 25's mach 3.2 speed was driven by the threat of the mach 3.0 B-70 nuclear penetration bomber program.  The US cancelled the XB-70 program but the Soviets kept the MiG25 alive.

It's important to note though, that out-running a missile doesn't necessarily mean it's a race of who is faster. If I detect the missile launch at the outside of it's range, I can outrun the missile with a much slower aircraft then the missile flight time because I only have to reach the edge of it's range. This is much less of a factor then your post would seem to suggest in the development of modern aircraft (which are still designed to be able to defeat these missiles - that have only gotten faster as you mention).

@DanNeely I'd say that should be the primary answer.  The MiG-25 and F-14 were both primarily bomber interceptors.  The F-14 was designed around the avionics required to fire the AIM-54 Phoenix missile, which was designed in response to bomber threats.

The MiG-25 that was clocked at M3.2 was in a runaway engine-overspeed state, and the engines were wrecked. Pilots weren't allowed to exceed M2.5 without permission, and the sensible redline speed was M2.8.

One important factor is that the threat model changed as ICBM's replaced bombers as the primary delivery mechanism for nuclear weapons over the course of the 1960s.
Being a few seconds late in intercepting a bomber carrying several tons of conventional bombs isn't good and could easily result in a few hundred deaths.  There's a huge difference between that and being a few seconds late in stopping a bomber carrying nuclear weapons where a single bomb can kill tens or hundreds of thousands of people.
Of particular relevance to the your question the MiG-25's mach 3.2 speed was driven by the threat of the mach 3.0 B-70 nuclear penetration bomber program. The US cancelled plans to mass produce and deploy B-70 bombers but the Soviets kept the MiG25 alive.  While the MiG25 was first deployed a few years after the XB70 program was cancelled; the USA continued to build two XB-70's and used them for research purposes throughout the 60s.  The threat of a future supersonic bomber based on the research could not be counted out; and an interceptor significantly faster than the Mach 2 B58 would have remained a valuable part of their nuclear  defense plans until the B58 was retired in 1970.

If you are intercepting a bomber, it will be happening minutes at least from weapons are possibly launch. Despite huge reductions in the power of a nuclear blast by unit of distance increases, it isn't ideal to have them detonating near or being destroyed over your territory.
Simply too high a risk that your enemy may purposefully or inadvertently utilize nuclear weapons that upon destruction in an aircrash could release substantial levels of radiation into the environment, Powdered Beryllium or Plutonium certainly anti-personnel and long term in their ability to do harm.

@jCisco I'm not sure what your point is supposed to be.  Yes there will be a high probability of a small radological release from shooting down a nuclear armed bomber and a chance of a much larger one if the bombs detonate.  If you don't shoot the bomber down there's a 100% chance of a large release from the nukes detonating; and it will be directly on top of something important not just close to it.

Maximum speed is not the best speed to fight. Two major factors in a fighter must be taken into account:

Wing loading.
Thrust-to-weight ratio (T/W ratio).

Wing loading is the weight of the airplane distributed across the total area of the wing. This tells the aircraft's ability to turn sustained.
The T/W ratio is the amount of thrust developed by the engine divided by the weight of the aircraft, and this will tell the acceleration the aircraft is able to achieve in level flight. When both things are taken into account, you get a second parameter that is the aircraft ability to climb (climb rate).
When air war developed in the WW1/2, one rule of thumb for fighter pilots was that speed/altitude was life. If you examine the aircraft as a physical system of energy, you have two kinds of energy at your disposal: Kinetic energy (the actual rate of aircraft movement) and potential energy, the altitude of the aircraft. When you dive, you convert potential energy into kinetic energy. When you climb you turn kinetic energy into potential energy, until the point where your engine is the only thing that is inputting energy into the system (so you reach the stage where instantaneous climb rate ends and you start to see the sustained aircraft climb rate).
When your aircraft turns, it will be slowed a little bit, and the increase in induced drag generated by the turn will decrease your speed slowed. In the fighter pilot jargon, you are burning energy. There are two (again) kinds of turn, instantaneous and sustained.
The fighting styles where divided into two kinds: Turn and Burn and Boom and Zoom.
Turn and burn means that both fighters try to outturn each order in order to get into their six o'clock position in order to fire, it's a horizontal fight in circles. Boom and Zoom is a climb fight where the higher positioned aircraft dives into the other in order to make a pass, fire and then climb again in order to not lose airspeed (and thus energy).
Quickly the boom and zoom style was adopted. Experienced pilots only entered combat when sure to have higher altitude than their enemies, because altitude was a reservoir of energy to be exploited in combat.
A turn and burn styled fighter/pilot would quickly slow down and be unable to either attack or flee from a higher positioned aircraft.
So far so good.
After the war, all efforts went on to produce ever faster aircraft and aircraft able to reach high altitudes quickly. This has dual advantage of giving a better high altitude position and allowing such fighters to engage incoming bombers.
All changed when better missiles and onboard radars became available. Not that current fighters don't need to be able to dogfight, but because the missile became the major combat element. Two kinds of missiles come to my head at this stage. AAMs and SAMs.
The increasingly powerful radars (even onboard) meant that aircraft might very well attack things that are above it, meaning that the boom and zoom tactics might go wrong. Secondly, a radar allows one to see the enemy way before visual contact is made, giving more time to climb to a similar altitude, etc.
If you are flying very high, you will be seen on enemy radar far away, and missiles directed by those radars will come from various directions. To enter enemy airspace, you need to stay low, where air resistance is bigger and most aircraft can't reach the stated figures of speed. (See F-111 or Su-24 for example).
Bombers won't be flying at stratospheric levels like B-29 anymore, due to the same reasons. They need to enter enemy airspace at low altitude in order to have a chance of surviving. If they use standoff weapons, those will fly low too. So, if you are escorting a bunch of bombers in a mission, or you are trying to intercept those, they will not be very high.
When a onboard radar tries to lock against a low flying aircraft, there is a problem. Earlier radars where not able to do so at all. More modern versions (called look-down/shot-down) are able to lock a target against terrain, but not with the same performance as when they are looking up. Clutter (false echoes) is removed via software. Infrared detectors work better at high altitude (where air is colder and a hot fighter is more visible), than against the hot ground below, and so on.
So, all in all, fighter combat in intensive theaters will be going to be done at middle or low level, where speed is slower due to higher drag. Even then, all fighter aircraft have something called corner velocity, where the aircraft is able to sustain their most tight turns, and those speeds are usually far from being top speed.
Another factor is engine consumption. Only recently did supercruising aircraft came to be operational. Forth-generation aircraft where not able to break the sound barrier without using afterburns, which both increase aircraft detectability (due to increased heat signature) and fuel consumption.
The same aircraft that might travel 1000 km without using afterburners, might very well be unable to do more than 300 km while afterburning all along.
All things taken into account, an aircraft with more missiles, better radar and longer loiter time is preferred above fighters like the British Lightning or the F-104 which had a very high speed and climb rate, but were short-legged and lugged only a pair of missiles.

"called lock-down/show-down" - shouldn't that be "look down - shoot down"?

yes, editing...


text:
Is there an API to get real-time FAA flight data?
I have been using an Android app to track flights. Their information is pulled from their own proprietary database, and some (with 5 minute delay) from the FAA.
I was thinking about making an app that would do this as well by pulling from multiple data sources.
What are some good APIs, either paid or free, that gives you near realtime data of flying aircraft?

I went down your path a while back.  My conclusion is that there's already a perfectly good free service that does this aggregation: Flightradar24 and Flightaware.  Personally, Flightradar24 is a lot better at real-time data, but Flightaware seems to have better routes/flight plan data.  I also made my own setup for KORD (see Bret's link to http://aviation.stackexchange.com/questions/1510/atc-real-time-traffic-services).

You can't get real-time data from the FAA without an operational need, and organizations which do receive real-time data cannot legally re-distribute this data publicly, except to other organizations approved by the FAA. An operational need pretty much means you have to be a flight dispatcher for an airline or commercial operator, not just have an interest in tracking flights.
You can get some real-time data from your local area using an ADS-B receiver. See this  question for more information: Where can I find real-time flight tracking with the least amount of delay?.
If you are okay with the five minute delay, then flightaware has an API you might consider. The direct feed from the FAA is called Aircraft Situation Display to Industry (ASDI) and I believe you can apply for the delayed feed, though I've never gone through the process, and the government's site is very sparse in information. I would suggest contacting them before attempting to apply if it's something you're interested in.

The OpenSky Network has a free open-source API for real-time air traffic data. In its current version it allows users to retrieve live (and partially historical data). The data is retrieved by a network of ADS-B receivers and consists of

ICAO 24-bit transponder ID to identify the aircraft
the flight's callsign
aircraft's current location (latitude, longitude, altitude)
aircraft's current horizontal and vertical velocity plus heading
Unix timestamps which indicate the age of each field

It's a REST API which provides all these data in JSON. They also provide simple Java and Python bindings. Their official API documentation can be found here. The sources for the Java and Python bindings are on github.

@kepler22b I do not fully agree that it should be a comment, as it seems to answer the question. I would rather say that some more detail would be nice.

@Federico Added some more infos, thx for the hint

"The API lets you retrieve live airspace information for research and non-commerical purposes.
Please note that the API does not provide commmercial flight data such as airport schedules, delays or similar information that cannot be derived from ADS-B data contents!"


text:
How can the glide ratio in a balanced turn be estimated?
I am trying to estimate the bank angle/turn radius required to lose a given amount of altitude during a full orbit. This has led me to trying to estimate glide ratio in proportion to $L/D_{MAX}$ of an aircraft in a balanced gliding turn, knowing only $V_{MD}$ and $L/D_{1g}$ for $V_{MD}$.
My approach is to use the following relationships:
$D = D_0 + D_I$
$D_{0} = C_{D_0} 0.5{\rho}v^2 S$
$D_I = C_{D_I} 0.5{\rho}v^2 S$
$C_{D_I} {\alpha} {C_L}^2 $
$C_L {\alpha} L $
$L {\alpha} n $
$n = \frac{1}{cos({\phi})}$ (${\phi}$ = bank angle)
Assuming $D_0 = D_I$ at $V_{MD}$ I think I should be able to calculate:
$L/D_{n_g} = \frac{L/D_{1g}}{0.5 + 0.5(1/cos^2({\phi})}$
Since altitude lost in a full orbit becomes $\frac{2{\pi}r}{L/D}$ where r = turn radius and
$r = \frac{v^2}{g tan(\phi)}$
Altitude lost in a full orbit should ${\alpha} \frac{1}{cos^2({\phi}) tan({\phi})}$ which looks like this. Does my method look alright, or have I made any incorrect assumptions or errors?

Are you assuming no change in power from straight and level flight?

Skip, I'm assuming an unpowered glide. To keep the problem simple, I'm assuming the same total drag as in powered straight and level flight at speed for minimum drag (Vmd) (i.e. not considering windmilling/propeller drag).

Note, at the time I am writing this, I'm sitting here thinking that this should have had the title prepended with "Peter Kampf:". Then I scroll down and .... ;)

yeah, since I never understand his answers, I try to answer from a pilot's perspective, rather than an engineer's.

I get a variation of the glide ratio with $\frac{2}{\bigl(\cos\varphi + \frac{1}{\cos\varphi}\bigr)}$ for circles flown with a bank angle of $\varphi$ when compared to straight flight at the respective best speed for optimum L/D.
Your initial assumptions are right:
Lift: $L = c_L\,\rho\,\frac{v^2}{2}\, S\, n_z$
Load factor: $n_z = \frac{1}{\cos\varphi}$ with the bank angle $\varphi$
Drag: $D = D_0 + D_i = \Bigl(c_{D0} + \frac{c_L^2}{\pi\, AR\,\varepsilon}\Bigr)\,\rho\,\frac{v^2}{2}\, S$ (zero-lift drag + induced drag), and both are equal at optimum $L/D$:
$$\Bigl(\frac{L}{D}\Bigr)_{\rm opt} = \Bigl(\frac{c_L}{c_D}\Bigr)_{\rm opt} = \frac{\pi\, AR\,\epsilon}{2\, c_L}\text{ for straight flight at }1\, g \text{ ($n_z = 1$)}.$$
Now with load factor $n_z = n$ and unchanged speed:
$$\frac{L_{ng}}{D_{ng}} = \frac{\frac{c_{L_{1g}}}{\cos\varphi}}{\frac{c_{L_{1g}}^2\bigl(1 + \frac{1}{\cos^2\varphi}\bigr)}{\pi\, AR\,\epsilon}} = \frac{\pi\, AR\,\varepsilon}{c_{L_{1g}} \bigl(\cos\varphi + \frac{1}{\cos\varphi}\bigr)}.$$
The ratio between both glide ratios is $\frac{2}{\bigl(\cos\varphi + \frac{1}{\cos\varphi}\bigr)}$, which makes sense because it will become 1 for $\varphi = 0$ and become less than 1 for nonzero values of $\varphi$. Note that the glide ratio at n g doesn't carry an "opt" subscript anymore, because by increasing $c_L$ we move away from the optimum. If the best L/D would be maintained for circling flight, speed would need to increase, not lift coefficient.
The altitude loss in straight flight of a distance equal to the diameter of a circle with the radius $R = \frac{v^2}{g\tan\varphi}$ is $2\,\pi\, R \, \frac{D}{L}$. When flown at the correct bank angle, this altitude loss per circle $\Delta h$ will become
$$\Delta h =  \frac{\pi\, v^2}{g \tan\varphi} \Bigl(\frac{D}{L}\Bigr)_{1g}\Bigl(\cos\varphi + \frac{1}{\cos\varphi}\Bigr).$$
Plot of the glide ratio with bank angle divided by the glide ratio for straight flight (blue line). I also added the altitude loss for a full circle (red line, Basis: $L/D = 18$ at 100$\,$m/s in straight flight), both for flight at constant speed and constant lift coefficient:

Use the results from this plot with caution at higher bank angles: Lift would need to double at a bank angle of 60°, doubling $c_L$, so we fly much more slowly than any pilot would. The red line gives more realistic values, since here the aircraft speeds up for flying higher bank angles. Flying at constant speed gives less altitude loss simply because the circle becomes smaller than when flying at constant $c_L$. Note that for every meter flown the altitude lost is greater when flying at constant speed, indicated by the blue glide ratio degradation curve.
Two factors conspire to increase aerodynamic losses when flying at constant speed with higher bank angles:

You need to create more lift, which causes higher induced drag, and
Your optimum $L/D$ is at a higher speed when the bank angle increases, which again increases aerodynamic losses because you move away from the optimum.

If you try to express the altitude loss per circle $\Delta h$ as a function of $\varphi$, you will get a quadratic equation and two solutions, one high and one low bank angle:
$$\frac{\cos^2\varphi + 1}{\sin\varphi} = \frac{g\,\Delta h}{\pi\, v^2} \Bigl(\frac{L}{D}\Bigr)_{1g}.$$
Maybe MathSE can solve this algebraically; I would resort to iteration or tabulation here.

Big thanks for the comprehensive answer Peter! I wonder: since ratio between two glide radiuses (L/D) vary with 2/(cos(x)+(1/cos(x)), shouldn't (D/L) actually vary with the inverse of that?

@Waked: You are soooo right! Sorry, I messed up. When you click on the edit history, you will see that I got it initially wrong from the beginning and had to change the fraction into its inverse. But I forgot the big, bold equation at the end of the post ....

No worries, I'm very grateful for your answer! Is there any way I can solve this to get the bank angle as a function of altitude loss per circle, (given constant v and L/D1g)? It seems to be beyond my skills (algebraically, save for solving it iteratively).

@Waked: Not easily, because there are two bank angles for some altitude loss values. I have updated the plot to include the altitude loss (red line). Give me some more time ...

@PeterKämpf I improved some of your math formatting, I hope you don't mind. Me personally, even if there were an algebraic solution to the equation, I wouldn't use it here, I think it's important to see the plot of the characteristics, not just the mathematically optimal point.

@yo', I agree that the plot offers a great insight and a good learning experience. My particular application calls for solving the specific navigational problem of calculating the minimum bank angle required for a given altitude loss at a low cost in terms CPU cycles. I am however getting decent results solving it iteratively!

@PeterKämpf sorry to wake this up again, but I believe there's a typo with the equation for height lost/orbit, above the graph. I am unable to replicate your plot. No constants, x from 0 to PI/2, markings every 22,5 degrees: http://fooplot.com/plot/c4dbbevyy8. Running the numbers for hand for 20 and 80 degrees give me 980 and 186 m respectively (100m/s, L/D 18). To my amusement, using the equation I posted in the question to calculate L/D tracks your graph perfectly :), must be the same trigonometric identity I guess.

@Waked: You are right, I screwed up. But in the plot! I used $v^2$/$\cos\varphi$ instead of straight $v^2$, so the lift coefficient in the plot would be constant, but the speed would increase with bank angle. I added the correct plot!


text:
What is the equation for the energy balance between turbine, compressor and propeller?
Let's consider the next turboprop arrangement:

What I am looking for
I am looking for an equation that involves $\dot{W}_T$, $\dot{W}_C$ and $\dot{W}_P$ (i.e. the power done by each component). This needs to be done through an energy balance in the shaft.

What we know
We know that the shaft has an efficiency of $\eta_{\text{shaft}}$, and the gearbox has an efficiency of $\eta_{\text{box}}$.

My try (#1)
Since energy is derived from the turbine, we adopt the convention $\dot{W}_T>0$, therefore $\dot{W}_P, \dot{W}_C < 0$. In other words, energy is required to move the compressive devices.
Following the definition of mechanical efficiency: $$\boxed{\eta_{\text{shaft}}=\dfrac{-\dot{W_C}-\dot{W_P}}{\dot{W_T}}}$$
However, I don't know how to incorporate the efficiency $\eta_{\text{box}}$ into this equation. This led me to try and find another expression.

My try (#2)
Following an hydraulic analogy for the power, I thought that whatever power the turbine generates needs to be invested in:

Increasing the temperature of the bearings of the main shaft (because the shaft is not ideal and has an efficiency of $\eta_{\text{shaft}}$).
Moving the compressor.
Increasing the temperature of the gearbox (because the box is not ideal and has an efficiency of $\eta_{\text{box}}$).
Moving the propeller.

Following this thinking, I can intuitively find this next equation (which I don't know if is correct or not): $$\boxed{\eta_{\text{shaft}}\times \dot{W}_T = -\dot{W}_C-\dfrac{\dot{W}_P}{\eta_{\text{box}}}}$$
The thing is I don't know if I assigned the efficiency $\eta_{\text{box}}$ correctly or not. Is there an easier way (i.e. less intuitive/more mechanical way) to apply the energy equation? What if we had 2 or more shafts?

The shaft is generally pretty stiff and doesn't directly have moving parts. So I assume by shaft efficiency you are thinking about bearing losses?

Exactly OSUZorba. That's the energy lost due to friction in the bearings (unconveniently used to increase its temperature )

Since the compressor loses a fraction of the turbine's work through the shaft, but is unaffected by the gearbox, but the prop experiences losses from the shaft and the gearbox, this equation should be what you need:
$$\dot{W_t}=\frac{-\dot{W_c}}{\eta_{shaft}}+\frac{-\dot{W_p}}{\eta_{shaft}*\eta_{gearbox}}$$
Which is the same as your attempt two (with some algebra).
As you add more efficiencies to a component, you would just continue to multiple the all $\eta$'s together to get the total efficiency of that component, since efficiencies are multiplicative and the associative property of multiplication.

Happy to see that our solutions agree!